1. Field of Invention
The field of the currently claimed embodiments of this invention relates to high dynamic range (DR) radio frequency (RF) power monitoring devices and methods.
2. Discussion of Related Art
Accurate knowledge of the RF specific absorption rate (SAR) in the body during magnetic resonance imaging (MRI) scans is important for patient safety and compliance with limits mandated by the Food and Drug Administration (FDA) in the USA1 and the International Electro-technical Commission (IEC) in Europe.2 In addition to ensuring safe operation and regulatory compliance, accurate power monitoring can avoid restrictions on clinical MRI sequences arising from incorrect estimation of the delivered power. Accurate knowledge of delivered power is essential for testing the MRI safety of peripheral, implanted and interventional devices at defined RF exposure levels.3-6 
RF safety concerns initially arose with the introduction of higher-field 1.5 Tesla (T) whole-body MRI scanners and the recognition that SAR increases approximately with the square of MRI frequency or field-strength when other MRI sequence parameters are kept constant.7-9 The recent emergence of clinical 3 T scanners and experimental body systems operating at 7 T and higher,10 in which SAR could potentially increase 4- to more than 20-fold compared to 1.5 T, only exacerbates concerns about safety and how to ensure compliance with SAR guidelines.1,2 
In clinical MRI scanners, SAR monitoring for safety and regulatory compliance is generally carried out by scanner software and hardware that is largely proprietary, with “scanner SAR” values typically logged for each study. These systems prohibit or terminate scanning based on predictions of body SAR relying on internal measures, modeling, and prior characterization or assumed properties of the MRI transmit coil. Electromagnetic modeling with knowledge of the input power 11-13 and thermal mapping14,15 can help provide a detailed understanding of whole body and local SAR. Yet, rare as they may be compared to the total number of MRI scans performed, RF burns do occur, a fraction of which are reported to the FDA.16 In these cases, at least, a failure in scanner SAR monitoring has occurred.
Unfortunately, investigating whether the scanner is operating safely within SAR guidelines by means that are independent of the scanner, if performed at all, is not easy.17 The accuracy of scanner SAR estimates is also questionable in light of discrepancies with thermally-derived SAR measurements,17,18 especially during MRI safety-testing of interventional devices3, 18-20 and the lack of correlation between subjective heat perception by patients and scanner SAR.21 
Setting precise SAR exposure levels for investigators testing devices or MRI methods, or for evaluating SAR in individual burn cases,22 requires accurate and independent measurement tools. This starts with accurate measurements of the total power deposited and requires a reliable RF power meter. The RF power monitors built into the MRI scanner are usually attached to the RF power amplifier output. However, measuring the power delivered to the body is complicated by losses in the RF transmission chain, including the cables, switches, the quadrature-hybrid (Q-hybrid) and the MRI coil.23,24 These losses can vary over time, but are not routinely monitored.
Moreover, as we now report, the very high dynamic range (typically >20 dB) (DR=peak-to-average power ratio) of RF transmit pulses, and MRI duty cycles that span orders-of-magnitude, are beyond the capabilities of available commercial power meters.25 These meters are adequate for pulse sequences with short repetition periods (TR) and consistent high power levels. However, they do not give accurate results for sequences using mixtures of high and low amplitudes or modulations, or long TR. There thus remains a need for improved systems and methods for measuring high dynamic range, low duty cycle (typically <1%) RF power.